You need to be more specific here:
How did you modify your BIOS, what memory is on your GPU (Hynix/Elpida/Samsung)?
What are your other settings aside of mem clock (core clock, core voltage, mem voltage) ?
My rig is running at an average of 725 H/s/card (lowest card ~690H/s, fastest ~760H/s) so i can't help you much if your target is to increase the hashrate to 900 H/s :p